The yeast t-RNA tyr genes number eight: SUP 2, 3, 4, 5, 6, 7, 8 and 11. Seven of these have been isolated by recombinant DNA cloning in phage lambda gt. One of these seven has been identified as the SUP 4 gene, by studies of the DNA from a yeast strain disomic for chromosome 10 (on which SUP 4 maps) and by meiotic mapping of an inherited difference in the hybridization pattern of t-RNA tyr to agarose gels of yeast EcoR1 fragments. The nucleotide sequence of DNA present at the SUP 4 locus will be determined for this gene cloned both from SUP 4 plus and SUP 4 UAA yeast strains. The cloned DNA will also be used to identify tyrosine t-RNA precursor molecules made in the yeast nuclear transcription system. Recombinant DNA cloning of the structural genes for yeast iso-1 and iso-2 cytochrome c is also in progress. These genes will be used for transcription studies and DNA sequencing.